There have been recent increase in reported knife crime throughout the United Kingdom, while social media usage has also increased. Existing research has reported associations between social media usage and secondary traumatic stress (STS), with symptoms similar to that of posttraumatic stress disorder (PTSD). The current study is the first to investigate the relationship between the frequency of viewing knife crime-related news via social media and STS. This study additionally investigated potential moderating effects of gender, age, and residential location. Participants (N = 155) completed an online survey which included the Secondary Traumatic Stress Scale (STSS), modified to refer to knife crime. Participants also self-reported the frequency of viewing knife crime on social media and relevant demographic information. Hypotheses and data analysis plan were pre-registered. 63% of participants reported STS symptoms, and 28% reported possible clinical levels of PTSD. A higher frequency of knife crime viewed via social media was significantly associated with higher STS symptoms, and this study found evidence that this effect is moderated by residential location, with those participants living in towns and villages most at risk. Younger, female participants scored highest on the STSS overall. It is important to understand the impact of viewing knife crime content via social media on mental health, and identification of those most at risk of experiencing STS will enable targeted intervention strategies.